Bidirectional electronic switch and dimmer comprising a light emitting device to illuminate a photo-activated electronic device

ABSTRACT

A novel approach for the control of AC power uses power MOSFETs in a bidirectional switch subcircuit configuration having an optically coupled, electrically floating control circuit that self-biases the switches into the “on” state and uses an optically coupled control element to force the switches into the “off” state. The time constant of the control circuit is fast enough to allow phase control as well as on-off control. A plurality of subcircuits can be easily cascaded to provide improved performance.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.16/092,839 filed Oct. 11, 2018 titled ELECTRONIC SWITCH AND DIMMER bythe same inventors and currently pending.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

BACKGROUND OF THE INVENTION Technical Field

The invention relates to a power management system and methods toprovide an electronic switch and dimming control.

Related Background Art

Traditional access to alternating current (AC) electrical power in homeand business environments is provided by mechanical outlets that arewired into the facility electrical system. These outlets are protectedfrom excessive electrical loads or potentially dangerous ground faultsusing electromechanical devices such as fuses and circuit breakers.Similarly, the control of conventional electrical room appliances suchas lighting and ceiling fans occurs using electromechanical switches.These fundamentally mechanical control devices provide simple on-offcontrol and inevitably wear out and, over time, can cause short circuitsor potentially dangerous arcing.

More nuanced control of common electrical appliances is typicallyprovided by electronic devices such as triacs which allow the AC mainswaveform to be interrupted on a cycle-by-cycle basis, so-called phasecontrol. Although significantly more efficient than the rheostats orautotransformers that preceded them, triacs are still too inefficient tobe used effectively in small enclosures for the control of largeelectrical loads and can induce electrical noise back into the facilityelectrical system.

Thus, there is a need for an improved electronic control system thatprovides a wider range of more reliable and highly efficient controloptions for broad application in facility electrical systems.Furthermore, there is a need for such a control system that can berealized using semiconductor devices that can be integrated with othercircuitry for advanced power control functions that can be manufacturedat low cost.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to a novel approach for the control of ACpower throughout a facility electrical system ranging from simple outleton-off switching to continuous variation of the applied AC power for,for example, the dimming of electrical lights. More particularly theinvention relates to a combination of functions that provides in oneembodiment both on-off and phase-control of the AC mains waveform.

One embodiment uses power MOS field-effect transistors (MOSFETs) aselectronic switches having very low “on” resistance connected betweenthe AC mains supply and the desired load. Since typical power MOSFETsintrinsically incorporate a body diode in parallel with the conductingchannel, pairs of devices are connected in a back-to-back arrangementhaving the source terminals in common to provide a truly bidirectional(AC) switch configuration. In order to control the switching action ofthe power MOSFETs a novel floating control circuit is employed that usesrectifying diodes connected at the drains to precharge the gate-sourcebias voltage thereby turning both devices “on”, and an optically coupledphototransistor that shorts the gate terminals to the common sourceterminal to force the devices into their “off” state when illuminated byan isolated optical source. Thus, the power MOSFET switches are normally“on” unless forced “off” by the optical control signal. The opticalcontrol signal can be applied continuously for nominal on-off control ofthe power delivered to the load, or it can be synchronized with the ACmains waveform to provide phase control. Integrated control circuitryfor the optical control signal can provide either leading edge phasecontrol preferred for switching reactive loads or trailing edge phasecontrol preferred for nonlinear loads such as LEDs. The specificexamples are not intended to limit the inventive concept to the exampleapplication. Other aspects and advantages of the invention will beapparent from the accompanying drawings and detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the basic power MOSFET bidirectionalswitch unit.

FIG. 2 is a schematic diagram of a prior art bidirectional switch usingoptoelectronic bias generation.

FIG. 3 is a schematic diagram of the basic elements of the improvedbidirectional switch.

FIG. 4 is a schematic diagram of an embodiment of the improvedbidirectional switch.

FIG. 5 is a schematic diagram of the embodiment of FIG. 3 using twoswitching elements to reduce total switch “on” resistance and increasetotal switch “off” resistance.

FIG. 6 is a schematic diagram of an embodiment similar to that of FIG.3, but with the switching elements in both arms of the AC power supply.

FIG. 7 is a schematic diagram of the embodiment of FIG. 5 using fourswitching elements to further reduce total switch “on” resistance andfurther increase total switch “off” resistance.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic diagram showing the basic power MOSFETbidirectional switch controlling the power delivered from AC source 101to load 108. Power MOSFETs 102 and 103 include body diodes 104 and 105,respectively. Switch 106 controls the gate-to-source bias voltageapplied to power MOSFETs 102 and 103. In the “on” position bias voltage107 is applied to the gate terminals of the power MOSFETs. Voltage 107is a voltage greater than the threshold voltage of the power MOSFETs(typically 5 to 10 volts) causing an inversion layer to form therebycreating a conducting channel extending from the drain to the source ofeach device. In this “on” state, the drain-to-source behavior of eachpower MOSFET can be modeled as a low value resistor, R_(ds). As long asthe voltage drop between drain and source remains below about 0.6 volt,the body diodes remain nonconductive and can be neglected. In the “on”state the circuit of FIG. 1 is equivalently the load 108 connected to ACsource 101 through a series resistor having value 2R_(ds).

In the “off” position of switch 106 the gate terminals of the powerMOSFETs are shorted to the source terminals and the drain-to-sourceconducting channels vanish as long as the drain-to-source voltageremains below the breakdown voltage of the body diodes. In the “off”state the circuit of FIG. 1 is equivalently the load 108 connected to ACsource 101 through back-to-back body diodes 104 and 105, whicheffectively disconnects the load 108 from source 101.

The requirement that the drain-to-source voltage of the power MOSFETsremain below the breakdown voltage of the body diodes, V_(br), in the“off” state requires that the breakdown voltage of the body diodesexceed the peak voltage of AC source 101. Thus, for example, assumingthat source 101 corresponds to a common 120 volt (rms) AC mains, thenthe breakdown voltage of each body diode must exceed the peak sourcevoltage of 170 volts.

A more detailed analysis of the power MOSFET structure shows that thebody diode is effectively the base-collector junction of a bipolartransistor connected in parallel with the MOSFET channel. Additionalparasitic elements include the capacitance of the base-collectorjunction and a parasitic resistance between the base and the emitter.This AC-coupled circuit places a constraint on the rate of change of thedrain-to-source voltage, dV_(ds)/dt, to avoid forward biasing thebase-emitter junction, thereby causing the bipolar transistor to conductwhile the MOSFET channel is “off”. While the resulting leakage currentmay not be sufficient to energize the load 108, it may be large enoughto cause additional efficiency or safety concerns.

Similarly, consideration of the constraints in the “on” state requirethat the drain-to-source voltage drop for each power MOSFET given byR_(ds)*Iload be less than about 0.6 volts. Potentially more important isthe power dissipated in each power MOSFET in the “on” state given byR_(ds)*Iload² which must remain less than a few watts to avoid excessivetemperature rise. Thus, for example, switching a common householdcircuit from a 120 volt AC mains having a typical limit of 20 amperesrequires that R_(ds) for each power MOSFET be less than 0.005 ohms (5milliohms.)

It is well known in the art that the breakdown voltage of the body diodecan be advantageously traded off against the value of R_(ds) by varyingthe structure and the doping levels in the device. In particular, it hasbeen shown that the value of R_(ds) is proportional to V_(br) ^(2.5).Thus, for example, cutting V_(br) in half results in reducing R_(ds) bya factor of 5.7. The circuit of FIG. 1 shows that the conceptual biasswitching circuit comprising switch 106 and voltage source 107 floatselectrically with the common source terminals of the back-to-back powerMOSFETs 102 and 103 which vary across the entire peak-to-peak range ofsource 101. Although simple in concept, this circuit can be difficult torealize in practice at low cost.

FIG. 2 shows a schematic diagram of a prior art approach to the controlcircuit. Voltage source 106 in FIG. 1 is replaced with a photovoltaicdiode stack 201 that provides the needed gate-to-source bias voltagewhen illuminated by a light emitting diode (LED) 206 which is powered bya separate low voltage source 203 and controlled by switch 204 throughcurrent limiting resistor 205. Elements 203-206 are assumed to be withinoptical proximity of diode stack 201. When LED 206 is switched off, thevoltage across diode stack 201 is drained through resistor 202 and thepower MOSFETs enter the “off” state.

Although the circuit of FIG. 2 works for simple on-off switchingapplications, the time constants associated with charging anddischarging the gate-to-source capacitance of the power MOSFETs throughthe bias circuitry are typically too large to effect phase control in50/60 Hz AC mains.

FIG. 3 is a schematic diagram showing the basic elements of the improvedswitch circuit. Although power MOSFETs are the preferred embodimentswitching devices discussed in the following description, it will beapparent to one skilled in the art that other types of field-effecttransistors can be advantageously employed in the improved circuit. Asin FIG. 1, voltage 107 is used to bias power MOSFETs 102 and 103 intotheir “on” state. Opposite to the operation of the circuit in FIG. 1,the power MOSFETs are “on” only as long as switch 106 remains open. Whenswitch 106 is closed the power MOSFETs are forced to enter their “off”state since their gates and sources are shorted together and voltage 107is dropped across resistor 300.

FIG. 4 is a schematic diagram showing an embodiment of the inventivecircuit. Voltage source 106 in FIG. 1 is replaced in switching unit 400with a Zener diode 402 having a Zener voltage greater than the thresholdvoltage of the power MOSFETs. Zener diode 402 is biased throughrectifier diodes 404 and 406 connected at the drain terminals of thepower MOSFETs and protected by current limiting resistors 403 and 405,respectively. Thus, in the absence of illumination resistor-diodebranches 403-404 and 405-406 provide bias for Zener diode 402 wheneither of the drain terminals exceeds the Zener voltage, placing powerMOSFETs 102 and 103 in the “on” state. When illuminated by LED 206phototransistor 401 shunts the bias current from branches 403-404 and405-406 to the source terminals of the power MOSFETs placing them in the“off” state. In this circuit the turn-on time constant is dictated bythe value of the current limiting resistors 403 and 405 and thegate-to-source capacitance of the power MOSFETs, while the turn-off timeconstant is dictated by the saturation current of the phototransistor401 at the illumination level provided by LED 206. Both of these timeconstants can be designed to be much shorter than the period of the ACmains, thereby allowing this embodiment to operate in both an on-off anda phase-control mode.

FIG. 5 is a schematic diagram of the embodiment of FIG. 4 using twoswitch units 400 to improve the performance of the circuit. In thisembodiment it is assumed that the power MOSFETs are selected to havehalf the breakdown voltage of the units used in FIG. 4. Thus, the onresistance of the individual switch units can be expected to be reducedby a factor of 5.7, as described above, and the total on resistance ofthe two switch units connected in series is reduced by a factor of 2.8relative to the circuit in FIG. 4. Additionally, the voltage drop acrosseach of the switch units in the “off” state is halved, thereby reducingthe dV_(ds)/dt experienced by each unit by a factor of two andconsequently reducing the “off” state leakage current.

FIG. 5 also includes an electronic switch circuit to control theillumination of LED 206. The current through LED 206 from voltage source203 is limited by resistor 205 and is controlled by transistor 500.Transistor 500 is controlled by an external control voltage applied tocontrol terminals 501. This allows for the rapid switching of the LED insynchronism with the AC mains waveform through external controlcircuitry (not shown) to provide phase control of the applied ACwaveform, as is used in dimmer applications. In another embodiment thecontrol signal is a train of pulses synchronized with the AC mainswaveform and having adjustable pulse widths to effectively control theaverage current/power delivered to the load, thereby providing a dimmingeffect for a light source load and a speed control for an AC motor load.In another embodiment the control signal is a train of pulses having afixed or variable frequency independent of the AC mains waveform therebygenerating a radio-frequency (RF) power waveform at the load terminalsfor use as a wireless charger/generator. In another embodiment thecontrol signal is a variable DC voltage allowing variable illuminationof the LED thereby allowing the MOSFETs to operate in a linear mode.

FIG. 6 is a schematic diagram of an embodiment similar to that of FIG.5, but with an individual switch unit 400 placed in each arm of the ACpower supply. The inventor has found that this circuit configurationfurther improves the turn-off characteristics of the switch devices,further reducing leakage currents.

FIG. 7 is a schematic diagram of the embodiment of FIG. 6 using twoswitch units 400 in each arm of the AC supply to further improve theperformance of the circuit. In this embodiment it is assumed that thepower MOSFETs are selected to have one-fourth the breakdown voltage ofthe units used in FIG. 3. Thus, the on resistance of the individualswitch units can be expected to be reduced by a factor of 32, asdescribed above, and the total on resistance of the two switch unitsconnected in series is reduced by a factor of 8 relative to the circuitin FIG. 4. Additionally, the voltage drop across each of the switchunits in the “off” state is quartered, thereby reducing the dV_(ds)/dtexperienced by each unit by a factor of four and consequently furtherreducing the “off” state leakage current relative to the circuit in FIG.4. As mentioned above, the inventor has found that this circuitconfiguration further improves the turn-off characteristics of theswitch devices, further reducing leakage currents.

SUMMARY

A novel approach for the control of AC power throughout a facilityelectrical system is described. The approach uses power MOSFETs in abidirectional switch subcircuit configuration having an opticallycoupled, electrically floating control circuit that self-biases theswitches into the “on” state and uses an optically coupled controlelement to force the switches into the “off” state. The time constant ofthe control circuit is fast enough to allow phase control as well ason-off control. A plurality of subcircuits can be easily cascaded toprovide improved performance.

We claim:
 1. A bidirectional electronic switch circuit having an inputterminal and an output terminal and further comprising: a. first andsecond series connected electronic switch devices, each switch devicehaving a drain terminal, a source terminal and a gate terminal and beingcharacterized by a threshold voltage specified between the gate terminaland the source terminal, wherein the drain terminal of the first switchdevice comprises the input terminal of the switch circuit and drainterminal of the second switch devices comprise the output terminal ofthe switch circuit, the source terminals of the first and second switchdevices are interconnected at a first control terminal and the gateterminals of the first and second switch devices are interconnected at asecond control terminal; b. a voltage source having a voltage thatexceeds the switch device threshold voltage and applied across the firstand second switch device control terminals through a current limitingresistor; c. a switch connected across the first and second devicecontrol terminals; and d. a first and a second bidirectional electronicswitch control terminals; and e. wherein the switch comprises: aphoto-activated electronic device characterized by a conductanceproportional to the intensity of illumination incident upon thephoto-activated electronic device and connected from the first switchdevice control terminal to the second switch device control terminal;and f. a light emitting device connected to the first and the secondbidirectional electronic switch control terminals and arranged toilluminate the photo-activated electronic device wherein the intensityof the light emitted by the light emitting device is proportional to anamplitude of an external control signal applied to the first and secondbidirectional electronic switch control terminals.
 2. The bidirectionalelectronic switch circuit of claim 1 wherein the first and secondelectronic switch devices are MOSFETs.
 3. The bidirectional electronicswitch circuit of claim 1 wherein the voltage source comprises: a firstrectifier device connected from the input terminal of the switch circuitto the second switch device control terminal; a second rectifier deviceconnected from the output terminal of the switch circuit to the secondswitch device control terminal; and, a voltage regulator deviceconnected from the first switch device control terminal to the secondswitch device control terminal; wherein the rectifier devices comprisefirst and second semiconductor diodes each having anode and cathodeterminals, wherein the anode terminal of the first semiconductor diodeis connected to the input terminal of the bidirectional switch circuitthrough a current-limiting resistor, the anode terminal of the secondsemiconductor diode is connected to the output terminal of thebidirectional switch circuit through a current-limiting resistor, andthe cathode terminals of the first and second semiconductor diodes areconnected to the common gate terminal of the switch devices.
 4. Thebidirectional switch circuit of claim 1 wherein the voltage sourcecomprises: a first rectifier device connected from the input terminal ofthe switch circuit to the second switch device control terminal; asecond rectifier device connected from the output terminal of the switchcircuit to the second switch device control terminal; and, a voltageregulator device connected from the first switch device control terminalto the second switch device control terminal; wherein the voltageregulator device comprises a semiconductor Zener diode having an anodeterminal and a cathode terminal, wherein the anode terminal is connectedto the common source terminal of the switch devices and the cathodeterminal is connected to the common gate terminal of the switch devices.5. The bidirectional switch circuit of claim 1 wherein thephoto-activated device comprises a semiconductor phototransistor havinga collector terminal and an emitter terminal, wherein the emitterterminal is connected to the common source terminal of the switchdevices and the collector terminal is connected to the common gateterminal of the switch devices.
 6. The bidirectional switch circuit ofclaim 1 wherein the light emitting device comprises a semiconductorlight-emitting diode having an anode terminal and a cathode terminal,wherein the anode terminal is connected to the first control terminal ofthe bidirectional switch circuit and the cathode terminal is connectedto the second control terminal of the bidirectional switch circuit.
 7. Amethod of using the bidirectional electronic switch circuit of claim 1to couple AC power to a load device comprising: a. first and secondpower input terminals for receiving power from an AC source; b. firstand second power output terminals for providing AC power to the loaddevice; c. connecting the input terminal of said bidirectionalelectronic switch circuit to the first power input terminal and theoutput terminal of said bidirectional electronic switch circuit to thefirst power output terminal; d. connecting the second power inputterminal to the second power output terminal; and e. providing anelectronic control signal to the first and second control terminals ofthe bidirectional electronic switch circuit.
 8. The method of claim 7wherein the control signal applied to the first and second controlterminals of the bidirectional electronic switch circuit is pulsed insynchronism with the AC power source to provide phase control of the ACpower coupled to the load.
 9. A method of using the bidirectionalelectronic switch circuit of claim 1 to couple AC power to a load devicecomprising: a. first and second power input terminals for receivingpower from an AC source; b. first and second power output terminals forproviding AC power to the load device; c. first and second circuitarrays each comprising a plurality of bidirectional electronic switchcircuits arranged in a series configuration wherein the input terminalof the first bidirectional electronic switch circuit is the inputterminal of the circuit array and the input terminal of each succeedingbidirectional electronic switch is connected to the output terminal ofthe previous bidirectional electronic switch except that the outputterminal of the last bidirectional electronic switch circuit is theoutput terminal of the circuit array, all of the first control terminalsof the bidirectional electronic switch circuits are interconnected toform the first control terminal of the circuit array and all of thesecond control terminals of the bidirectional electronic switch circuitsare interconnected to form the second control terminal of the circuitarray; d. connecting the input terminal of said first bidirectionalelectronic switch circuit array to the first power input terminal andthe output terminal of said first bidirectional electronic switchcircuit array to the first power output terminal; e. connecting theinput terminal of said second bidirectional electronic switch circuitarray to the second power input terminal and the output terminal of saidsecond bidirectional electronic switch circuit array to the second poweroutput terminal; and f. providing an electronic control signal to thefirst and second control terminals of the bidirectional electronicswitch circuit arrays.
 10. The method of claim 9 wherein the controlsignal applied to the first and second control terminals of thebidirectional electronic switch circuit is pulsed in synchronism withthe AC power source to provide phase control of the AC power coupled tothe load.
 11. The method of using the bidirectional electronic switchcircuit of claim 9 wherein the number of bidirectional electronic switchcircuits in the first and second circuit arrays is selected on the basisof the peak voltage of the AC power source.
 12. The method of using thebidirectional electronic switch circuit of claim 9 wherein the controlsignal is a train of pulses synchronized with the AC mains waveform andhaving adjustable pulse widths to effectively control the averagecurrent/power delivered to the load, thereby providing a dimming effectfor a light source load and a speed control for an AC motor load. 13.The method of using the bidirectional electronic switch circuit of claim9 wherein the control signal is a train of pulses having a fixed orvariable frequency independent of the AC mains waveform therebygenerating a radio-frequency (RF) power waveform at the load terminalsfor use as a wireless charger/generator.
 14. The method of using thebidirectional electronic switch circuit of claim 9 wherein the externalcontrol signal applied to the first and second bidirectional electronicswitch control terminals is a variable DC voltage allowing variableillumination of the light-emitting device thereby allowing theelectronic switch devices to operate in a linear mode.